This 503 error message has been giving me a serious headache while riding my Cannondale Moterra 2018 with the Bosch Performance CX motor. Then I found out, that when the 503 error occurs, I could use the original speedsensor magnet (which is now permenantly mounted on the bike!) to "freeze up" the speedsensor by just waving it past the speedsensor in a rythmic fashion. You can see on the onboard computer display, that the speedsensor sends a signal to the computer calculating the speed of the bike. Once the computer gives you a speed-reading, just turn the bike off and on again - and you shoud be good to go!
